The corrosion protection of steel substrates is an important industrial process. This process is important for many industries including the automotive industry, steel industry and so forth. Currently the most common methods of corrosion protection of steel substrates are galvanizing, zinc phosphate, electrodeposition of organics, conventional spray or dip priming, oil coating and so forth. However, in the automotive industry and other high performance uses these methods have the following problems: (1) pollution in the form of Volatile Organic Content (VOC), (2) excessive waste disposal, (3) inadequate coverage of recessed areas, and (5) inadequate long term corrosion protection performance.
It is known that plasma deposition of thin films gives a very dense layer of film, with uniform deposition, no "pin holes", and good edge coverage. Furthermore this process does not use solvents so there is no VOC problem. Most of the work in the plasma deposition area has been restricted to small objects (e.g. microelectronic components). Plasma processing for larger objects has been used primarily for plastic substrates.
It has been disclosed that it is possible to deposit thin polymer films on metal substrates. The plasma deposition of organic films on metal is generally described in an article entitled "Surface Coating of Metals in a Glow Discharge" in the Journal of the Oil and Colour Chemists Association, Vol. 48, 1965 (hereinafter, the "Glow Discharge" article) This article describes in general terms a method of coating a steel substrate with thin polymer films derived from organic vapors (styrene, acrylates, butadiene, diethyl silicate, and tetraethyl orthosilicate) using glow discharge (i.e. plasma deposition) for short term protection of the steel substrate. The process utilizes AC power and system pressures of 1-5 torr.
Professor Yasuda disclosed some basics of plasma energy level, the deposition of organosilanes, and oxygen cleaning of the steel substrate in an article entitled Plasma Polymerization at Combined Energy Input For Protective Coatinq of Metal, published in the "Journal of Applied Polymer Science: Applied Polymer Symposium 42, 233 (1988). However, this article discloses using only an AC power source.
Professor Yasuda also disclosed the use of magnetrons in plasma polymerization in the article Plasma Polymerization by Maonetron Glow Discharge. I.Effect of Magnetic Filed on Breakdown of Monomers in Low Pressure, published in J. Vac. Sci. Technol. A 7 (2), Mar./Apr. 1989. However, this article also discusses only AC power not DC power. When magnetrons were used in DC power systems it was previously believed that magnetrons could function only as a cathode and not as an anode (See, Thin Film Processes, edited by Vossen and Kern, 1978, page 76, first paragraph).
The use of polysiloxanes as the deposited film in glow discharge for corrosion protection of steel is also disclosed in Japanese Patent No. Sho 51 [1976]-83030 issued to Aoki (hereinafter the Aoki patent). This disclosure also teaches the use of AC power. It is not believed that the Aoki patent technology would have practical utility as a long term corrosion protection method for automotive steel because of adhesion problems.
What is needed is a method of depositing a thin organic film layer on a variety of different steel substrates to give improved corrosion resistance. The method must result in a film having good adhesion, good edge coverage, good barrier properties and the method must have minimal VOC problems, minimal deposition on the cathode and chamber walls and minimal waste disposal.